SparseMatrix in Eigen

Question

If I set the value of the SparseMatrix record in Eigen as follows:

sparse_matrix->coeffref(10, 10) = 0;

Will this actually reduce the amount of memory required by the matrix, or will he try to save 0 and use 4 bytes there (assuming an integer type)?

if the answer is the last, how to set the columns to 0 so that it does not use extra space?

Also, something like this:

typedef Eigen::Triplet<double> TripletType;
std::vector<TripletType> t;
for (int i = 0; i < some_value; ++i) {
    for (int j = 0; j < some_value; ++j) {
        t->push_back(TripletType(i, j, 0);
    }
}
sparse_matrix->setFromTriplets(t);

Will this lead to explicit zeros in the sparse matrix?

Answer 1

After pasting with, coeffrefyou can crop the sparse matrix, for example:

Eigen::SparseMatrix<double, Eigen::ColMajor> A(5,5); // fill A A.insert(0,0)=9.; A.insert(1,0)=3.0/2.0; A.insert(0,1)=3.0/2.0; A.insert(2,0)=6.0; A.insert(0,2)=6.0; A.insert(3,0)=3.0/4.0; A.insert(0,3)=3.0/4.0; A.insert(4,0)=3.0; A.insert(0,4)=3.0; A.insert(1,1)=1.0/2.0; A.insert(2,2)=12.0; A.insert(3,3)=5.0/8.0; A.insert(4,4)=16.0; std::cout << A << std::endl; std::cout << A.data().size() << std::endl; A.coeffRef(3,0) = 0; A.prune(0,0); // Suppresses all nonzeros which are much smaller than reference under the tolerence epsilon std::cout << A << std::endl; std::cout << A.data().size() << std::endl;

Output:

Nonzero entries:
(9,0) (1.5,1) (6,2) (0.75,3) (3,4) (_,_) (_,_) (_,_) (1.5,0) (0.5,1) (6,0) (12,2
) (0.75,0) (0.625,3) (3,0) (16,4)

Outer pointers:
0 8 10 12 14  $
Inner non zeros:
5 2 2 2 2  $

9 1.5 6 0.75 3
1.5 0.5 0 0 0
6 0 12 0 0
0.75 0 0 0.625 0
3 0 0 0 16

16
Nonzero entries:
(9,0) (1.5,1) (6,2) (3,4) (1.5,0) (0.5,1) (6,0) (12,2) (0.75,0) (0.625,3) (3,0)
(16,4)

Outer pointers:
0 4 6 8 10  $

9 1.5 6 0.75 3
1.5 0.5 0 0 0
6 0 12 0 0
0 0 0 0.625 0
3 0 0 0 16

12

You can see that the size has changed from 16 to 12, since three are also removed (_,_).

sizeof(), .